Wafers used for Advanced Wafer Level Packaging (aWLP) come in a variety of sizes and formats. For example, the thicknesses of aWLP wafers range from tens of microns to a few millimeters. Additionally, the wafers may be mounted on film frames, bonded to carrier wafers, or reconstituted onto packaging materials or other wafers. This diversity results in significant variation in wafer stiffness and warp, both of which adversely affect the ability of wafers to be fixed to a conventional chuck.
Conventional chucks use a vacuum to fix a wafer to the chuck. This method of fixation tends to preserve any warping in the wafer, as the wafer is drawn to the surface of the chuck without having an opportunity to flatten out through transverse extension of the warped portion of the wafer. Thus, a wafer fixed to a chuck in this way may not conform to the surface geometry of the chuck. Physical edge clamps which clamp the wafer directly to the chuck can improve the fixation of the wafer to the chuck by forcing the wafer to conform to the surface geometry of the chuck, however, physically clamping a wafer may generate debris particles or other contaminants.
As the optical inspection of wafers requires establishing a focus plane with high precision, warping in the wafer can cause portions of the wafer to be outside the desired focus plane. Further, the debris particles generated by physical clamping can contaminate the die, which in turn may negatively affect yield.